The invention relates to a method of manufacturing a. piezoelectric filter with an acoustic resonator comprising a layer of a piezoelectric material which is provided with an electrode on either side, which resonator is situated on an acoustic reflector layer formed on a surface of a carrier substrate.
Such a piezoelectric filter, also referred to as xe2x80x9cThin Film Acoustic Wave Resonatorxe2x80x9d or xe2x80x9cSolidly Mounted Bulk Acoustic Wave Resonatorxe2x80x9d, can particularly suitably be used in electronic circuits employed in equipment for personal wireless communication, such as GSM telephony. For such applications, it is desirable to use filters of small dimensions with a quality factor Q above 1000 and a resonant frequency in the range between 500 MHz and 5 GHz.
The layer of piezoelectric material may be, in practice, for example a layer of aluminium nitride AlN or zinc oxide ZnO. These layers are provided in a thickness which is equal to half the wavelength with which acoustic waves of said frequencies propagate in these materials. The acoustic reflector layer on which the resonator is situated, in practice, generally consists of several layers of alternately a high and a low acoustic impedance. Said layers customarily comprise, for example, tungsten with a comparatively high acoustic impedance of approximately 100 Gg/m2s and, for example, silicon oxide or a synthetic resin with a comparatively low acoustic impedance of, respectively, approximately 13 Gg/m2s and approximately 2 Gg/m2s. These layers are provided in a thickness which is equal to a quarter of the wavelength with which acoustic waves of said frequencies propagate in these materials. For frequencies in said frequency range, both the piezoelectric layers and the layers of the reflector have thicknesses in the range from 1 to 3 xcexcm. A layer of tungsten can form both an electrode of the resonator and a layer of the acoustic reflector.
In U.S. Pat. No. 5,873,154 a description is given of a method of the type mentioned in the opening paragraph, wherein first an acoustic reflector is formed on a surface of a carrier substrate of, for example, glass, silicon or gallium arsenide, whereafter an acoustic resonator is formed on this reflector. The resonant frequency of the above-described filters is 1 GHz. In one of the examples, the reflector, viewed from the carrier substrate, is composed of an approximately 1.5 xcexcm thick layer of silicon oxide and an approximately 1.3 xcexcm thick layer of tungsten, and in another example the reflector is composed of an approximately 0.5 xcexcm thick layer of a polymer, such as polyimide, and an approximately 1.3 xcexcm thick layer of tungsten. In both examples, the acoustic resonator comprises an approximately 1 xcexcm thick layer of zinc oxide.
A drawback of the known method resides in that materials for the layers of the resonator and the acoustic reflector layer cannot be chosen freely and independently. In this known method, the layer of piezoelectric material must be formed at a relatively low temperature. During the formation of this layer, care must be taken not to damage the layers already present on the surface of the carrier substrate. In the second example, where the layer of piezoelectric material is deposited on a layer of polyimide, the temperature of the carrier substrate must not exceed, for example, 350xc2x0 C. In practice it has also been found that a layer of tungsten having a thickness as mentioned hereinabove may become locally detached from the carrier substrate when it is heated to a temperature of 350xc2x0 C. This means that in practice the temperature at which the piezoelectric layer can be formed is limited to approximately 300xc2x0 C. It may however be desirable to deposit this layer at a higher temperature. To deposit a crystalline layer of piezoelectric material wherein the crystals are equally oriented, it is often necessary to heat the carrier substrate during the deposition to a temperature above 300xc2x0 C. To obtain such a crystalline layer of aluminium nitride it is desirable, for example, to heat the carrier substrate during the deposition to a temperature between 400 and 800xc2x0 C.
It is an object of the invention to provide a method which enables the materials for the layers of the resonator and the acoustic reflector to be chosen freely and independently.
To achieve this, the method is characterized in accordance with the invention in that the layer of piezoelectric material is provided on a surface of an auxiliary substrate, after which a first electrode is formed on the layer of piezoelectric material, the acoustic reflector layer is provided on and next to the first electrode, and the structure thus formed is secured with the side facing away from the auxiliary substrate on the carrier substrate, after which the auxiliary substrate is removed and a second electrode situated opposite the first electrode is provided.
A customary auxiliary substrate of, for example, glass or silicon can be heated to the above-mentioned high temperatures without any problems during the deposition of the layer of piezoelectric material. After the deposition of the layer of piezoelectric material, an electrode and an acoustic reflector layer are formed on the layer. This can be carried out without causing damage to the layer of piezoelectric material. Customary layers for the electrodes and the acoustic reflector layers can be deposited without the temperature of the auxiliary substrate exceeding 350xc2x0 C.
Preferably, there is started from an auxiliary substrate in the form of a slice of silicon, which is provided at its surface with a layer of silicon oxide, after which the layer of piezoelectric material is deposited on this layer of silicon oxide, a first electrode is formed on the layer of piezoelectric material, the acoustic reflector layer is provided on and next to the first electrode, and the structure thus formed is secured on the carrier substrate with the side facing away from the auxiliary substrate, whereafter the auxiliary substrate is removed and the exposed layer of silicon oxide is removed at the location of the second electrode to be formed, after which the second electrode is provided. By virtue of the presence of the layer of silicon oxide, a properly oriented layer of crystalline piezoelectric material can be formed in practice, while the auxiliary substrate can also be readily removed because the layer of silicon oxide can be used as a layer on which an etch process stops automatically. In an etch bath comprising potassium hydroxide, silicon can be very selectively etched with respect to silicon oxide.
After the removal of the auxiliary substrate, the second electrode can be readily provided on the layer of piezoelectric material then exposed. While this second electrode can be readily contacted externally, the first electrode cannot be contacted as easily; this requires the formation of a window through the layer of piezoelectric material. Preferably, the second electrode is embodied in the form of two adjacent sub-electrodes, which are both situated opposite the first electrode, so that a piezoelectric filter with two series-connected resonators is formed between the two sub-electrodes. As a result, the filter can be readily contacted externally on the two sub-electrodes. The first electrode is not connected.
A simple construction is further obtained if the auxiliary substrate is secured to the carrier substrate by means of a layer of an adhesive which forms part of the acoustic reflector layer, in particular if the sub-electrode of the acoustic resonator also forms part of the acoustic reflector layer. In practice, this is feasible because a layer of adhesive has a relatively low acoustic resistance and electrode materials have a much higher acoustic resistance. The thickness of the sub-electrode must be chosen so as to be in accordance with the desired resonant frequency of the filter, while the adhesive layer may be much thicker on account of its very low acoustic impedance.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.